On_delete options (CASCADE, PROTECT, SET_NULL) in Django - Step-by-Step Execution

Concept Flow - On_delete options (CASCADE, PROTECT, SET_NULL)

Delete Parent Object

↓

Check on_delete option

↓

CASCADE

↓

End

When a parent object is deleted, Django checks the on_delete option to decide what happens to related child objects.

Execution Sample

Django

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

# Deleting an Author triggers on_delete behavior on Book

This code shows a Book linked to an Author. Deleting the Author triggers the on_delete rule on Book.

Execution Table

Step	Action	on_delete Option	Effect on Child Object	Result
1	Delete Author object	CASCADE	Delete all related Book objects	Books deleted automatically
2	Delete Author object	PROTECT	Prevent deletion if related Book exists	Error raised, Author not deleted
3	Delete Author object	SET_NULL	Set Book.author to NULL if allowed	Author deleted, Books remain with author=NULL
4	Delete Author object	Other (e.g. DO_NOTHING)	No automatic action on Book	Possible DB error if FK constraint violated
5	End	-	-	Process complete

💡 Process ends after applying the on_delete rule to child objects.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Author Exists	True	False (CASCADE)	True (PROTECT)	False (SET_NULL)	Depends on option
Book Count	5	0 (CASCADE)	5 (PROTECT)	5 (SET_NULL, author=NULL)	Depends on option
Book.author FK	Author ID	N/A (deleted)	Author ID	NULL	Depends on option

Key Moments - 3 Insights

Why does PROTECT prevent deleting the parent object?

PROTECT raises an error if child objects exist, stopping deletion. See execution_table row 2 where deletion is blocked.

What happens to child objects with SET_NULL if the FK does not allow null?

If FK disallows null, SET_NULL causes a database error. The child FK must allow null for SET_NULL to work, as shown in execution_table row 3.

Does CASCADE delete only direct children or deeper related objects too?

CASCADE deletes direct children and can cascade further if those children have on_delete CASCADE. The example shows direct children deleted at step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what happens to Book objects when Author is deleted with CASCADE?

A Books are deleted automatically

B Books remain unchanged

C Books' author field is set to NULL

D Deletion is blocked with an error

💡 Hint

See execution_table row 1 under 'Effect on Child Object'

At which step does deletion fail due to PROTECT?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Check execution_table row 2 where error is raised

If Book.author FK does not allow null, what happens with SET_NULL on delete?

A Books are deleted

B Author deletion is blocked

C Books' author field is set to NULL

D No effect, deletion proceeds

💡 Hint

See key_moments question 2 about FK nullability and SET_NULL

Concept Snapshot

Django on_delete options control child object behavior when parent is deleted:
- CASCADE: deletes child objects automatically
- PROTECT: blocks deletion if children exist
- SET_NULL: sets FK to NULL if allowed
Choose based on desired data integrity and app logic.

Full Transcript

In Django, when you delete a parent object linked by a ForeignKey, the on_delete option decides what happens to related child objects. CASCADE deletes all related children automatically. PROTECT stops deletion and raises an error if children exist. SET_NULL sets the child's foreign key to NULL if the field allows nulls. Other options exist but these three are common. This visual shows step-by-step what happens when deleting a parent with each option, how variables like object existence and foreign keys change, and common beginner questions about behavior and errors. Understanding these helps keep your database consistent and avoid unexpected deletions or errors.